In recent years considerable research has been devoted to the development of magnetostrictive compounds, and in particular rare earth-iron alloys. These developments are summarized by A. E. Clark, Chapter 7, pages 531-589, in "Ferromagnetic Materials," Vol. 1 (Ed. E. P. Wohlfarth, North-Holland Publ. Co., 1980). A major objective of the research has been to develop rare earth-iron alloys with large room temperature magnetostriction constants. Technically important alloys having these properties include alloys of terbium together with dysposium and/or holmium. The relative proportions of the rare earths and the iron are varied to maximize room temperature magnetostriction and minimize magnetic anisotropy. Presently, the most technically advanced alloy of this kind is represented by the formula Tb.sub.x Dy.sub.1-x Fe.sub.1.5-2.0 wherein x is a number from 0.27 to 0.35. An optimized ratio is Tb.sub.0.3 Dy.sub.0.7 Fe.sub.1.9 which is known as terfenol-D, as described in U.S. Pat. No. 4,308,474.
Such rare earth-iron alloys are true compounds and can exist in crystalline or polycrystalline form. In preparing elongated bodies (viz. rods) from such alloys, grain-orientation of the crystals is essential for achieving high magnetostriction. An axial grain orientation of the crystallites not only increases the magnetostriction constant but also reduces internal losses at the grain boundaries. This is particularly important in applications where a high magnetostriction at low applied fields is required. (See Clark, cited above, pages 545-547).
U.S. Pat. No. 4,609,402 of O. Dale McMasters describes a sequential process for casting magnetostrictive rods, and thereafter subjecting the rods to zone melting and recrystallization to obtain an axial grain orientation. In the casting process, a hollow mold tube is positioned with its lower end portion within a molten body of the alloy contained in a crucible. A pressure differential is created between the chamber enclosing the crucible and the mold tube so that the alloy melt is forced upwardly through the bottom of the mold tube to a selected level for molding an elongated rod. A portion of the molten alloy is left in the crucible so that solid particles of higher melting impurities present in the alloy collect at the surface of the melt remain within the crucible as the rod is cast from the subsurface melt. After solidification, the rod is removed from the mold tube, and in a separate operation is subjected to a free-standing zone-melting, and recrystallization, to produce an axial grain orientation.
Although the method of the McMasters patent is capable of producing high quality magnetostrictive rods, there has been a need for a more continuous casting and crystallization process adapted to larger volume production. At the same time, however, it has been desired to maintain the impurity-separation advantage of the McMasters method, and also to achieve at least as effective axial grain orientation as with the McMasters method. Prior to the present invention, these related objectives had not been satisfactorily achieved.